1. Field of the Invention
The present invention relates generally to the formation of semiconductor particles, and more specifically to the formation of nanocrystalline semiconductor particles.
2. Description of the Background Art
Since the earlier studies, starting in mid-1980's, various synthetic approaches have been developed in making nanosized II-VI (Zn and Cd chalcogenides) and IV-VI (Pb chalcogenides) semiconductors. Much of this effort is aimed at achieving a very narrow distribution in the size of the particles. The basic idea is to use the spatial or chemical confinement provided by matrices or organic capping molecules to terminate the growth of nanocrystallites at any desired stage. In most cases, lack of a microscopically uniform environment in the substrates might be the cause for relatively wide size distribution. Both organic and inorganic matrices, such as monolayers, polymers, inverse micelles, and zeolites have been used to control the particle size. Recently, other researchers have synthesized monodispersed CdSe nanocrystallites based on the pyrolysis of organometallic reagents. This approach uses the concept of Ostwald ripening for size selective precipitation of nanocrystallites. So far, intensive efforts have been made to synthesize quantum-sized II-VI semiconductors especially on the CdS.sub.x Se.sub.1-x systems, while much fewer reports exists on IV-VI (PbE, E.dbd.S, Se, Te) compounds. Research interest in IV-VI semiconductors arises because they are small band gap materials with greater quantum-size effect and larger optical nonlinearity compared to II-VI materials.
Conventional wet chemistry synthesis performed without matrix assistance normally results in the production of micron size particles. Various host matrices, such as glass, zeolites, sol-gels, and micelles, have been used to synthesize nanoparticles. However, a number of problems are associated with these methods. The particles synthesized in glasses and sol-gels exhibit large polydispersity, since they are not ordered structures. Even though the particles formed in ordered zeolites are mostly monodispersed, the pore sizes available in zeolites are limited to below 14 .ANG.. Another disadvantage with these methods is the inability to easily isolate the nanoparticles from the matrix material. In the case of micelles, even though it is possible to isolate the particles, large scale manufacturing becomes prohibitively expensive due to the low precursor concentrations required.
Many surfactants, when mixed with water, self-assemble into a mesoporous phase with long range three-dimensional periodicity called the Bicontinuous Cubic Phase (BCP). A wide spectrum of surfactants which can form cubic phase when mixed with water or nonaqueous solvents in binary or ternary systems can be easily found in Fontell, Coll. Poly. Sci. 1990, 268, 264-285, the entirety of which is incorporated herein by reference for all purposes. There have been a number of publications (Luzzati et al., Nature 1967, 215, 701; Lindblom et al., J. Am Chem Soc., 1979, 101, 5465; Larsson, L., J. Phys. Chem. 1989, 93, 7304-7314; Linblom et al., Biochim. Biophys Acta 1989, 988, 221-256) on structure and diffusion properties of bicontinuous cubic phase of lipids and other surfactants. As defined by Lindblom et al., supra, a bicontinuous cubic phase is a "crystalline lipid/water phase . . . in which the lipid aggregates to form a three-dimensional lattice" (page 225, col. 1) and which has "regions which are continuous with respect to both polar (water) and nonpolar (hydrocarbon) components" (page 228, col. 2, emphasis added). FIG. 1 shows the Im3m structure of the bicontinuous cubic phase 10. Unlike inverse micelles which form isolated water pools encircled by lipid molecules, aqueous pores 12 in the cubic phase are interconnected with neighboring ones through small channels (not shown), with sizes ranging from 2-10 nm depending on type of surfactant and amount of water. Surfactant bilayers 18 form the exterior boundaries of aqueous pores 12. Due to the bicontinuous nature of cubic phase 10, ions can diffuse from pore to pore in the hydrophilic aqueous region without passing through the surfactant membrane barrier.